This invention relates to a method of electrolytically etching aluminum foil particularly for use as aluminum electrodes or foils for electrolytic capacitors. More particularly, this invention relates to a method of electrolytically etching aluminum foil in which both alternating current (AC) and direct current (DC) electrolytic etching are employed. Further this invention relates to aluminum foil produced by the method of the invention.
The purpose of etching aluminum foil particularly for use in electrolytic capacitors is to increase the effective surface of the foil as the capacitance of an electrolytic capacitor is proportional to the effective surface of the aluminum foils or electrodes employed in the capacitor.
During the etching process a certain amount of aluminum is removed from the foil. Because of this loss of aluminum frequently there is a mechanical weakening of the foil. This mechanically weakened foil, whose electrical characteristic is satisfactory for use in capacitors, has been found unsatisfactory for commercial production of electrolytic capacitors; particularly when the foil is required to have sufficient strength needed for the high speed winding of capacitor rolls employed in commercial practice.
Prior art electrolytic methods of etching aluminum have consisted of continuous or intermittent AC etching in acid baths for example as shown in Arora U.S. Pat. Nos. 4,315,806, and 4,376,686 or by DC etching as shown in U.S. Pat. No. 3,884,783. It is also known in the prior art to etch aluminum in an electrolytic solution in which a fluctuating unidirectional current is employed. This is as shown for example in the G.B. Pat. Specification No. 985024.
Japanese Kokai No. 78 19151 shows a combination of first electrolytically etching aluminum foil in an aqueous hydrochloric acid solution with AC, washing the foil to remove the hydrochloric acid from the foil, roasting the foil at 300.degree. C.-500.degree. C. and then further electrolytically etching the foil in an aqueous solution of hydrochloric acid solution employing DC at a current density of 1 amp/cm.sup.2 or 1.2 amps/cm.sub.2.
These known methods of electrolytically etching aluminum foil for use in electrolytic capacitors have not been too satisfactory as the resultant foils, after forming, do not exhibit a combination of desired capacitance and mechanical strength suitable for use in the commercial capacitor rolls capable of withstanding the high speed winding employed in commercial practice or if a desired combination of capacitance and mechanical strength is achieved it is accomplished by use of an expensive and time-consuming combination of steps, for example as set out in the aforementioned Japanese Kokai No. 78 19151.